Timely termination of cover crops is very important. USDA's Natural Resources Conservation Service and USDA's Risk Management Agency have updated their rules and guidelines for terminating or killing cover crops in fields that will be planted to corn and soybeans this spring. The guidelines use four strategic management zones across the nation, and Iowa has two of those zones. As the accompanying map shows, about a third of Iowa (western portion) is in Zone 3, while the remainder is in Zone 4.

TIME TO TERMINATE: Thousands of acres of cover crops were planted in Iowa last fall, many by farmers who have not had cover crops in the past. The time to terminate these cover crops is quickly coming up this spring, so having a plan in place will be beneficial.

These new guidelines for crop insurance and soil conservation purposes allow the cover crop to be terminated up to five days after planting (depending on where you are in the state—see the map), which allows planting into a green cover. However, when planting corn after cereal rye or triticale, it's generally recommended that the cover crop be terminated 10 to 14 days before planting to avoid the allelopathic effect of the winter cereals on corn. Soybeans typically are not affected by this.

If you have cover crops on your land, be sure to work on terminating the acres that will be planted to corn first, advises Mark Johnson, Iowa State University Extension field agronomist in central Iowa. Termination at least 10 days ahead of corn planting is best. Termination ahead of soybean planting is not as necessary. In fact, you can terminate all the way up to and including preemergence of the beans without concern for yield penalty, he says.

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The four common methods of terminating cover crops are: winterkilling, tilling, mowing, and applying herbicides. Each method has its disadvantages and limits, notes Johnson. For example, winterkill (the cover crop is terminated by a hard freeze) is only applicable to certain crops and climate regions; mowing is limited to certain cover crops and crop growth stages. Tillage can be expensive and can negate the benefits of the cover crops, as well as the benefits of minimum/no-till production systems.

Choosing a herbicide program to terminate a cover crop
Many factors also limit herbicides and herbicides may be completely prohibited in organic cropping systems. When selecting an herbicide program for termination of a cover crop, consider:

1) The cover crop species

2) The cover crop growth stage

3) Other weed species present

4) The production crop to be planted

5) The weather conditions at application

6) The type of herbicide used

Timely termination of winter grains used for cover is very important
Cover crops were one of the topics discussed at the annual On-Farm Network Conference sponsored by the Iowa Soybean Association in February at Ames. Trials completed by the On-Farm Network this past fall show that timely control of winter grains used for cover is very important. In the fall of 2012, several replicated strip trials were seeded with a drill after soybean harvest that compared untreated strips with strips seeded to Tillage Radish, Tillage Radish + oats, Tillage Radish + Fridge triticale, and Tillage Radish + Fridge triticale + crimson clover.

The spring of 2013 was extremely wet, making it difficult for the triticale to be terminated in a timely manner. Instead, the cover crops strips were terminated just prior to planting. In all three trials, both treatments with fridge triticale had a negative yield response. The TALLADEGA Mix, which included 41 pounds triticale seed per acre had a larger negative response on corn yield than the CHARLOTTE Mix, with only 22 pounds triticale seed per acre. (See table below.) In the same trials, strips with the tillage radish and the tillage radish annual oats averaged about a 5 bushel per acre yield increase when compared with the untreated strip. This data was presented by Tristan Mueller and Heath Ellison at the 2014 On-Farm Network Conference at Ames.

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See full reports for all 2013 cover crop trials from the On-Farm Network searchable trial database.

Triticale in these strips was coming back strong early last spring, just before the grower applied herbicide to control it. The strip on the right had been seeded with a mix that included triticale at 41 lbs. per acre, while the one on the left had only 22 lbs. of triticale per acre in the mix.

Cereal rye is the most common cover crop that will need to be terminated in the spring, but others like annual ryegrass and triticale may need to be terminated as well.

Because of weather and other issues during the 2013 crop year, only three of the trials (ST2013IA027A, ST2013IA029A and ST2013IA032A) seeded the fall of 2012 were completed successfully. The table shows the yield averages for strips in the three trials. In each case, there was a negative yield response when the cover crop seed mix included Fridge Triticale.

New crop insurance guidelines allow the cover crop to be terminated up to five days after planting (depending on where you are in the state—see accompanying map), which allows planting into a green cover. However, when planting corn after cereal rye or triticale, it's generally recommended that the cover crop be terminated 10 to 14 days before planting to avoid the allelopathic effect of the winter cereals on corn. The triticale and cereal rye strips in the three completed cover crop trials from 2013 seem to bear this out. Soybeans typically are not affected by this.

Other information on terminating cover crops can be found in a Purdue University "Terminating Cover Crops" publication.

Many growers who put out On-Farm Network cover crop trials last fall had mixes that contained annual ryegrass. Despite being called an annual, annual ryegrass can survive the winter. Be sure to check your fields early to determine whether it will need to be terminated this spring. This termination information from Cover Crop Solutions, sponsor of the 2012-2013 trials conducted by the On-Farm Network, should be helpful.